CROSS-REFERENCE TO RELATED APPLICATIONS
The assignee is a related company to the assignee of U.S. Pat. No. 4,864,536, issued Sep. 5, 1989 and entitled "Optical Memory Method and System", U.S. patent application Ser. No. 07/184,263, filed Apr. 21, 1988 now U.S. pat. No. 4,915,982 and entitled "Thin Film Photoluminescent Articles and Method of Making Same", U.S. patent application Ser. No. 07/225,846, filed Jul. 29, 1988 now U.S. Pat. No. 5,007,037 and entitled "Optical Disk Drive System", U.S. patent application Ser. No. 07/277,255, filed Nov. 29, 1988 and entitled "Three Dimensional Optical Memory". Reference is also made to commonly assigned U.S. patent application Ser. No. 07/449,002, filed concurrently herewith and entitled "Three Laser Optical Disk Drive System". Each of those disclosures is incorporated by reference herein.
The present invention relates to disks for use with optical mass storage devices for data storage. More particularly, the present invention relates to a method of and apparatus for constructing an erasable optical disk for mass data or information storage which performs all of the storage functions, that is, the writing, reading and erasing of data, in a purely photoelectric manner.
Optical storage devices for use in storing computer and other data are presently known in the art. Such devices are desirable because of their ability to store vastly more information per disk than known magnetic disk storage devices. While most of the known optical disk drive devices perform read-only functions, write-once-read-many times (WORM) and erasable optical memory systems are also known. However, heretofore, erasable optical memory systems have encountered much greater developmental difficulty than the read-only or WORM systems, due in part to the increased technical complexity of the characteristics of the disk media itself.
In order to overcome the problems of prior art erasable optical disk drives, a related company to the assignee of the present invention has developed a new approach to the optical storage materials which provide the storage function of the erasable optical disk drive. This development utilizes the phenomenon known as electron trapping in a class of materials which comprise an alkaline earth crystal typically doped with rare earth elements. Thin crystalline forms of such materials may be formed on various substrate structures, such as glass or alumina, in order to provide the disk storage medium. Since the electron trapping phenomenon is a purely electronic process, read-write-erase operations can be performed in very short periods of time. In addition, the physical trapping phenomenon appears to offer a practically limitless media life.
The materials which may be used as the media for the optical disk storage system described herein are the subject of U.S. patent application Ser. No. 07/184,263, which is a continuation of U.S. patent application Ser. No. 06/870,877, now U.S. Pat. No. 4,864,536 and U.S. Pat. No. 4,830,875, which issued on May 16, 1989. Other materials useful as the storage media herein are disclosed in U.S. Pat. No. 4,839,092, which issued Jun. 13, 1989 and U.S. Pat. No. 4,806,772, which issued Feb. 21, 1989 and U.S. Pat. No. 4,842,960, issued Jun. 27, 1989. The discloser of that application and of each of those patents is incorporated by reference herein.
Reference is made to each of the foregoing issued United States patents and patent applications, as well as to the applications discussed in the Cross Reference To Related Applications section for a discussion of the electron trapping and electron trapping media phenomena.
In constructing such disks suitable for use with electron trapping optical material coated on disks, it is desirable to utilize a disk structure which allows a sub-micron resolution of written data patterns. In prior disk structures, as disclosed in the Ser. No. 07/277,255 application and U.S. Pat. No. 4,864,536, for example, the methodology of writing, reading and erasing of the media consisted of performing all of those functions directly on a single layer of media. The data patterns are written in a specific location on the disk, using a focused short wavelength (for example, blue or green) light, which results in electrons becoming trapped and stored at those locations. The stored data patterns are then read by subsequent illumination with infrared light, which causes the trapped electrons to be released and to emit an orange light at the previously written storage locations. The orange emission is then converted to electrical signals which represent the original data patterns.
In addition to writing, reading and erasing functions, the necessary focusing and tracking functions are also performed using the same single layer electron trapping optical memory media. In order to accomplish the focusing and tracking functions, a portion of the infrared light produced by the read laser is reflected back from the media surface and transmitted to focusing and tracking error detection diodes. The derived focus error signal is proportional to the plane of the predetermined best focus of the reading laser spot and the active layer on the disk surface. The focus servo system utilizes that signal in order to position the objective lens so that the focal point is located at its optimum position.
The tracking operation is similar to that described with regard to the focusing operation, except that the output of the tracking error diodes is used to maintain both the radial position of the optical head on a given track and to permit the seeking and capture of a particular desired track.
Those prior disk structures for accomplishing reading, writing, erasing, focusing and track functions created certain disadvantages. For example, the assignee's prior methods of reading and writing would suffer light scattering, depending upon the materials utilized, due to internal reflections within the disk structure. The light scattering would cause regions of the material not illuminated by the write or read light sources to also participate in the electron trapping process. That effectively increases the size of the data mark. The enlargement of data marks due to internal scattering is referred to as "mark spreading".
Other disadvantages relate to the focusing and tracking functions. For example, the method for focusing and tracking has the potential for depleting the written data during the performance of the focus and tracking functions if the read beam illuminates and discharges data cells or bits. In addition, the prior method, while providing for the storage of permanent format and guidance data, did not make any provision to prevent accidental erasure of the stored data.
The invention disclosed herein overcomes the disadvantages of the prior disk structures and methods of performing the various functions associated with those structures by providing for several distinct layers each of which performs an active function, in addition to the active media layer. Incident light scattered to adjacent tracks is minimized by means of a reflective layer. The reflective layer is also utilized to carry the permanent format and guidance information as well as providing a surface for performing the focus and tracking functions. In addition, absorbing layers are included in order to attenuate reflections of light within the disk structure and to eliminate the spreading of marks.